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It depends on what hardwre you're using (VROM or VRAM). With VRAM, you write the data for the graphics into video memory. With VROM the graphics should already be in memory. If you're using sprites, you write the sprite info into sprite ram, or use normal ram and use a sprite-dma to put it into sprite ram. With backgrounds, you write to the 'nametables' to tell it what tile to display where. Then write to the 'attribute tables' to set the colors for the background tiles. Check the Nestech doc at http://nesdev.parodius.com for more info.